ext4: add "punch hole" flag to ext4_map_blocks()

This patch adds a new flag to ext4_map_blocks() that specifies the
given range of blocks should be punched out.  Extents are first
converted to uninitialized extents before they are punched
out. Because punching a hole may require that the extent be split, it
is possible that the splitting may need more blocks than are
available.  To deal with this, use of reserved blocks are enabled to
allow the split to proceed.

The routine then returns the number of blocks successfully
punched out.

[ext4 punch hole patch series 4/5 v7]

Signed-off-by: Allison Henderson <achender@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Mingming Cao <cmm@us.ibm.com>